fix "ns-render interpolate", so that it can render the input views, add crop box

[f-dy]

Previously, ns-render interpolate:

• Ignored distortion parameters
• Used an interpolated focal length, but principal point was from the first camera
• Rendered all images with the same size, even if input views were of different sizes
• Forgot to render the last frame
• There was no default support for crop box and background color, except when rendering from a camera path, where these would exclusively be fetched from the camera_path.json

Now:

• There is an command-line option --fixed-intrinsics to keep fixed intrinsics through the whole sequence, but they are all interpolated by default
• Everything is interpolated, even size, so that ns-render interpolate --interpolation-steps 1 should render exactly the input images
• The last frame is rendered
• All render modes have the options --use-bounding-box --background-color --bounding-box-min --bouding-box-max (same syntax as ns-export), which can override the json settings when rendering a camera path.

Works great with the gray colormap from https://github.com/nerfstudio-project/nerfstudio/pull/2058 to render object masks.

[tancik]

This is great! Can you post example videos before and after. Also how are the videos saved when the resolution changes?

[tancik]

Hi, just checking in on this PR, looking forward to it being incorporated.